The present invention relates generally to liquid ink recording apparatus or ink jet printers, and more particularly relates to a method and apparatus in such a recording apparatus for supporting and stacking liquid ink printed sheets.
Liquid ink printers of the type frequently referred to either as continuous stream or as drop-on-demand, such as piezoelectric, acoustic, phase change wax-based or thermal, have at least one printhead from which droplets of ink are directed towards a recording sheet. Within the printhead, the ink is contained in a plurality of channels. For a drop-on-demand printhead power pulses cause the droplets of ink to be expelled as required from orifices or nozzles at the end of the channels.
In a thermal ink-jet printer, the power pulses are usually produced by formation and growth of vapor bubbles on heating elements or resistors, each located in a respective one of the channels, which are individually addressable to heat and vaporize ink in the channels. As voltage is applied across a selected resistor, a vapor bubble grows in the associated channel and initially expels the ink therein from the channel orifice, thereby forming a droplet moving in a direction away from the channel orifice and towards the recording medium where, upon hitting the recording medium, a dot or spot of ink is deposited. Following collapse of the vapor bubble the channel is refilled by capillary action, which, in turn, draws ink from a supply container of liquid ink. Operation of a thermal ink-jet printer is described in, for example, U.S. Pat. No. 4,849,774.
The ink jet printhead may be incorporated into either a carriage type printer, a partial width array type printer, or a page-width type printer. The carriage type printer typically has a relatively small printhead containing the ink channels and nozzles. The printhead can be sealingly attached to a disposable ink supply cartridge and the combined printhead and cartridge assembly is attached to a carriage which is reciprocated to print one swath of information (equal to the length of a column of nozzles), at a time, on a supported, stationary recording medium, such as paper or a transparency.
After the swath is printed, the paper is stepped a distance equal to the height of the printed swath or a portion thereof, so that the next printed swath is contiguous or overlapping therewith. This procedure is repeated until an entire page is printed. In contrast, the page width printer includes a stationary printhead having a length sufficient to print across the width or length of a supported sheet of recording medium at a time. The supported recording medium is continually moved past the page width printhead in a direction substantially normal to the printhead length and at a constant or varying speed during the printing process.
In either case, the substrate or sheet is supported on a supporting assembly that comprises a platen. Typically, the sheet supporting platen consists of a flat surface, or of a rotating hollow drum, that in either case, has a back surface, and a front surface that has an area which is large enough to support up to a legal size sheet, with border areas left over. In some ink jet printers, the platen includes a heating device to attempt to dry the ink images. However, in a lot of small ink jet printers or systems including an ink jet printing step, there is no such heater.
Ordinarily however, as such ink jet printers are made faster and faster, the liquid ink images printed onto the supported sheet are usually not fully dry by the time the sheet exits the printer. Such wet images on exiting the printer will then to smear or offset if the exiting sheet comes into contact with a previously printed sheet in the output tray. Additionally, such printers are also being made smaller and smaller, thus providing less and less space for large trays or complicated pre-drop sheet handling mechanisms.
A known pre-drop sheet handling mechanism is disclosed for example in U.S. Pat. No. 4,794,859. The disclosed mechanism includes opposed movable rails thatt are associated with opposed walls with no vertical sheet guides which can lead to sheet misaligment; each rail includes a return spring, a pressurable wing member, a pivot on the floor of the sheet support tray outwardly of a sheet support area thus requiring a relatively large tray floor; and a recess in a wall for receiving the rail in an opened position, all in all making the mechanism bulky, complicated, and costly.
There is therefore a need for a relatively small, simple and low cost system that can handle such sheets with wet or likely wet images so as to prevent such undesirable smearing or ink offset.